In response to the demand for more reliable and higher capacity data storage and retrieval systems, there is considerable activity in the research and development of so-called optical disk recording systems. These systems utilize a highly focused modulated beam of light, such as a laser beam, which is directed onto a recording layer which is capable of absorbing a substantial amount of the light. The heat thusly produced causes the light-absorbing material in the areas struck by the highly focused laser beam to change chemically and/or physically, thus producing a concomitant change in optical properties, e.g., transmissivity or reflectivity, in the affected area. For readout, the contrast between the amount of light transmitted or reflected from the unaffected parts of the absorbing layer and from the marked areas of the layer is measured. Examples of such recording systems are disclosed in U.S. patents throughout the literature and in numerous U.S. patents such as U.S. Pat. Nos. 3,314,073 and 3,474,457. In recording data, a rotating disk having a light-absorptive recording layer is exposed to modulated radiation from a laser source. This radiation is passed through a modulator and appropriate optics, and the highly focused laser beam is directed onto the disk which forms by chemical and/or physical reaction of the light-absorbing layer a series of very small marks along a circular path within the light-absorptive layer. The frequency of the marks is determined by the modulator inputs. Using laser beams with a focused spot diameter of 1 micron or less, data can be stored at a density of 10.sup.8 bits/cm.sup.2 or higher.
The simplest optical disk medium consists merely of a dimensionally stable solid substrate on which is coated a thin layer of light-absorptive material such as a metal layer. When the light-absorptive layer is struck by an intense beam of coherent light, such as from a laser source, the light-absorptive material is either vaporized and/or thermally degraded, thereby producing a very small marked area which exhibits different transmissivity or reflectivity than the adjacent unmarked layer. A more advanced laser recording medium is disclosed in Nam, U.S. Pat. No. 4,410,581 in which a single recording layer is completely encapsulated between an intermediate layer of solvent-resistant plastic material formed on a transparent substrate and a protective solvent-based plastic layer formed on the recording layer. In this instance, the encapsulated recording layer is imaged by a laser beam passing through the transparent substrate to burn a very small hole in the layer.
Multilayer antireflection structures, such as those disclosed in U.S. Pat. No. 4,305,081 to Spong and U.S. Pat. No. 4,270,132 to Bell, increase the absorption of the laser beam which also gives better read/write contrast than with the use of simple single layer media. Therefore, for purposes of obtaining better power efficiency, sensitivity and permanency of the record, it has been preferred to use multilayer antireflective structures.
There are two basic types of multilayer antireflective structures, one of which is basically a bilayer structure and the other a trilayer structure. In bilayer media, the substrate is coated with a very smooth, highly reflective material such as aluminum, on top of which is coated a layer of moderately light-absorptive material which is preferably of a thickness corresponding to about .gamma./4n, where is the wavelength of the recording light source and n is the refractive index of the light-absorptive layer. In trilayer media, the substrate is likewise coated with a first layer of very smooth highly reflective material on which is coated a second layer of transparent material. Atop the transparent second layer is coated a thin third layer of strongly light-absorptive material. The combined thickness of the transparent and absorptive layers is preferably adjusted to be about .gamma./4n. In both types of structures, the adjustment of certain layer thicknesses according to the wavelength of light and refractive index of the layer is for the purpose of minimizing the amount of light reflected from the unmarked areas and maximizing the amount of light reflected from the marked areas, thus producing a higher playback signal amplitude. A detailed discussion of the three types of disk construction is given by A. E. Bell in Computer Design, January 1983, pp. 133-146 and the references cited therein. See especially Bell and Spong, IEEE Journal of Quantum Electronics, Vol. QE-14, 1978, pp. 487-495.
It will be realized, of course, that the terms "bilayer" and "trilayer" refer only to the fundamental optical layers and do not exclude the use of ancillary layers. For example, a very thin layer of polymeric material may be interposed between the substrate and the reflective layer in order to compensate for insufficient smoothness of the substrate or to improve adhesion of the reflective layer. Also, one or more transparent layers may be coated over the light-absorptive layer to protect the fundamental layers from adverse atmospheric conditions or to insulate thermally the other layers. Also, quite frequently the light-absorptive layer will be coated with a relatively thick layer of transparent material which serves as a defocusing layer which prevents surface dust and contaminants from interfering with the optical properties of the entire medium.
While mark formation in such structures identified above frequently involves physical removal of material by ablation or some other means to form a pit or hole in the media, a dispersion imaging system such as disclosed in Hallman et al., U.S. Pat. No. 4,000,334 may also be used. In this instance, laser energy incident on a thin continuous absorptive layer, forms a transparent discontinuous area of dispersed small globules.
The desired properties of optical recording media are (1) high sensitivity, (2) high signal-to-noise ratio (SNR), (3) high tolerance to material variation, contaminants and other defects, and (4) high archival stability after extended storage and/or recording and readout (see Bartolini, J. Vac. Sci. Technology Vol. 18, No. 1, January/February 1981, p. 70). Based upon these criteria, a considerable amount of research has been and continues
to obtaining the best to be carried out directed possible disk materials. In particular, a majority of the work done up to this time on materials for the light-absorptive or recording layer has been directed to thin films of metals and chalcogenides such as tellurium and tellurium alloys, rhodium, bismuth, indium, lead, aluminum, platinum, nickel, titanium and silver. Of these, by far the greatest amount of work has been directed to the use of tellurium and its alloys with such elements as arsenic, antimony, selenium, germanium, phosphorus, silicon, thalium, indium, tin, copper, silver, iron, bismuth, aluminum, zinc and vanadium. While much effort has been spent in developing ternary and quaternary systems involving chalcogenides with metals, much less effort has been directed to binary systems such as tellurium/copper. Such a system has been disclosed in Japanese Patent Publication No. 60-208290 for use with laser radiation a 650 nm wavelength or longer. Inorganic oxides such as lead oxide, tungsten oxide, titanium oxide, silicon oxide, zirconium oxide and the like have also been investigated and found to be suitable to some extent as the recording medium for optical disks.
Many of the above metals, chalcogenides and their alloys form oxidatively stable films at thicknesses of several hundred angstroms or higher. However, when they are used as ultra-thin layers, e.g., below 150 A, as frequently required, the chalcogenides and their alloys are usually oxidatively unstable. Furthermore, chalcogenides and their alloys, especially Te and its alloys, frequently undergo extensive morphological changes with changes in temperature. While this change in morphology may be useful in some instances, more frequently it introduces undesirable element of instability.